Sinergismo in vitro tra rokitamicina e cotrimossazolo in S. pyogenes e S. pneumoniae

Background: Synergism between cotrimoxazole (SXT) and rokitamycin (ROK) was observed carrying out antimicrobial susceptibility tests on S. pyogenes and S. pneumoniae with the disk diffusion method (CLSI, 2005). The aim of this study was to confirm this phenomenon on a large number of isolates displaying different macrolide resistance phenotypes. Methods: Synergism between SXT and ROK on 104 S. pyogenes and 102 S. pneumoniae recently isolated was detected by a double-disk screening test.Time kill experiments were also performed on representative strains adopting standard procedures (CLSI 2005). Results: The combination of SXT plus ROK was synergistic against 93% S. pyogenes strains and 51% S. pneumoniae strains. On pneumococci SXT-S this percentage arise to 64%, while, on SXT-R it was 29%. In no instances antagonism was demonstrated. Synergism was not observed against S. pyogenes strains showing cMLSB phenotype. In S. pneumoniae no relationship between different mechanisms of macrolide resistance and the results of interactions was found. Results of time-kill experiments confirmed those obtained with double-disk assay in all the strains tested. Conclusion: Synergism between SXT and ROK was more frequently encountered among S. pyogenes than S. pneumoniae strains.This drug association may be synergistic or not when acting on different strains of the same bacterial species. Different macrolide-resistance mechanisms (reduced binding due to modification of the 50S subunit or efflux pump) among the various bacteria may eplain the observed differences.